In recent years, complementary metal-oxide semiconductor (CMOS) imagers have been increasingly widely used for digital still cameras, camcorders, surveillance cameras; the market thereof has also increasingly expanded. In such a CMOS imager, each pixel converts incident light to electrons by a photodiode, accumulating the electrons for a fixed period, and then outputs a signal in accordance with the accumulated charge amount to an analog to digital (AD) converter usually residing in a chip. The AD converter digitizes the signal, and then outputs the digitized signal to the following stage. In the CMOS imager, such pixels are disposed in the shape of a matrix for imaging.
A general pixel circuit includes a photodiode, a transfer transistor, a reset transistor, an amplification transistor, a floating diffusion region, a selection transistor, and the like. A photon incident into a silicon substrate of the pixel circuit generates an electron/hole pair; then the electron is accumulated in a node between the photodiode and the transfer transistor by the photodiode. The electrons are transferred to the floating diffusion region by switching ON the transfer transistor at predetermined timing to drive the gate of the amplifier transistor. Thus, signal charges become signals to vertical signal lines to be read through the selection transistor.
To the amplifier transistor and the vertical signal line, a fixed current circuit is connected. The fixed current circuit configures a source follower. A signal of a charge accumulation region is attenuated with a gain of slightly less than 1, and output to the vertical signal line.
Herein, in a general pixel circuit, one end of the reset transistor is connected to the gate of the amplification transistor through the charge accumulation region and the other end is connected to the source and the power supply of the amplification transistor. A row drive circuit draws out the electrons accumulated in the photodiode to the power supply by switching ON the reset transistor while simultaneously switching ON the transfer transistor and resets the state of the pixel circuit to a dark state before the accumulation (i.e., a state before light enters). As the voltage of the power supply, 3 V is supplied, for example.
In recent years, in such a CMOS imager, the parasitic capacitance in pixels is reduced due to miniaturization. Specifically, the parasitic capacitance of the floating diffusion region is remarkably reduced, resulting in improved conversion efficiency and improved sensitivity. In addition, the crystal quality of substrates have improved and a reduction in noise has been advanced. More specifically, the signal to noise (SN) ratio of signals has been remarkably improving. In view of such a tendency, a possibility of utilizing the CMOS imager as a photodetector for ultra low illuminance has arisen. For example, a photon counting image pickup device, in which the dynamic range has increased using time division and field division using a plurality of pixels in combination, has been proposed (for example, see Patent Literature 1.). Such a device may be used as a device for photon counting in which the entire pixel array in a chip is a single light-receiving surface; accordingly substitution for a photomultiplier and the like has been expected.
An image sensor employing such photon counting is free from a random noise and a fixed noise due to transmission and amplification of analog signals because data output from the pixels are treated as digital data from beginning to end. In this case, only a light shot noise and a dark current generated within the pixels remain. Particularly, in imaging with low illuminance, a dramatically high SN ratio can be obtained.